1. Field of the Invention
The present invention relates to a diffuser unit, a lithographic apparatus, a method for homogenizing a beam of radiation, a device manufacturing method and a device manufactured thereby.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and scanners, in which each target portion is irradiated by scanning the pattern through the beam of radiation in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In a lithographic apparatus it is important to have a good homogeneity of the intensity of the beam of radiation used therein. A better homogeneity, usually expressed as a uniformity value, allows a more constant quality of the details and structures which are obtained in the lithographic process. In the present context, uniformity relates to the variation of the received energy per area on the surface of the object to be illuminated, or more precisely, uniformity=(maximum dose−minimum dose)/(maximum dose+minimum dose). Herein, dose is the cumulated amount of received energy per area. Note that this may relate to overall uniformity, i.e. over all of a relevant area, or to local uniformity, i.e. over only part of the relevant area. Uniformity is preferably as low as possible, with an optimum of 0, i.e. a low value relates to good uniformity. Note that for lithography with a patterned beam of radiation, there are at least two objects which are illuminated, the patterning device and the substrate. Since the substrate is illuminated with a patterned beam, there will be inherent non-uniformities, and uniformity improvement will in this case most often relate to illumination of the patterning device. In all cases, it will be easy to identify the object or surface which is to be illuminated with a beam that is as uniform as possible.
In known lithographic apparatus there is often a built-in diffuser unit. Herein diffuser unit relates to a unit, constructed and arranged to make the beam of radiation more homogeneous, i.e. more uniform, before it strikes a surface to be illuminated, such as a patterning device (e.g. a mask). Known diffuser units are static. This means that they have a certain predetermined and constant influence on the distribution of radiation in the beam of radiation. Even though in theory, i.e. with an extremely constant radiation distribution in the beam of radiation, very good uniformity is obtainable, the known apparatus has the disadvantage that, when there is a slight change in the distribution, which may be due to various causes, the new distribution may be worse than before, even with the known diffuser unit. In other words, the known apparatus does not always offer the desired uniformity of the beam of radiation.